1. Field of the Invention
The invention concerns prevention of illegal copying generally and more specifically concerns the use of encryption and watermarking to prevent such copying.
2. Description of Related Art
Increasingly, works of authorship of the kinds protected by copyright are being encoded into digital representations and distributed in the digital representations. An example of such a digital representation is the one used for audio recorded on a compact disk. There are many advantages to digital representations: they can be made as faithful as desired to the original, they can be copied easily and precisely, they can be easily modified, and they can be easily decoded, and once decoded, they can be played, displayed, or printed. The copying, modification, and decoding can be done in any device which includes a microprocessor and memory, and the digital representations can be distributed by any of the techniques used to distribute digital data, including networks and storage devices such as diskettes, tapes, and CD-ROMs.
The extreme ease, rapidity, and precision with which digital representations can be copied and the ease and rapidity with which they can be distributed poses serious problems for authors. Digital representations of works are subject to the copyright law like any other representations, but there is serious concern that copying of digital representations of works will outpace the ability of the copyright law to prevent it. In response to this concern, a number of ways have been found of using technology to make illegal copying of digital representations more difficult. These ways fall into four main categories:
Controlling access to a digital representation so that those who do not have the author's permission do not have access to it; PA1 Controlling the uses that a person who has access to the digital representation may make of it; PA1 Encrypting the digital representation while it is being transmitted across the network and while it is stored on a storage medium or in memory, with decryption being possible only for an authorized person; and PA1 watermarking the digital representation, that is, providing it with markings that are invisible or inaudible when the digital representation is decoded and displayed or played but which may be read by machine. The markings indicate the source of the work.
The first three categories all have a common aim: to keep the digital representation of a work out of the hands of an unauthorized person. The techniques of these three categories are of no help once the digital representation "escapes", that is, falls into the hands of an unauthorized person in unencrypted form, since the unauthorized person can use the full panoply of digital methods to copy and distribute the escaped digital representation.
The watermarking technique of the fourth category does not interfere with copying, but does make it possible to determine the original source of the digital representation from the copy and therefore makes it easier to detect infringing copies and to determine their source. A general discussion of the problem of copyright protection for digital representations may be found in Eckhard Koch, Jochen Rindfrey, and Jian Zhao, "Copyright Protection for Multimedia Data", in: Digital Media and Electronic Publishing, Academic Press Ltd., 1996.
An example of a system for controlling copying that uses the first three techniques can be found in U.S. Pat. No. 5,646,999, Saito, Data Copyright Management Method, issued Jul. 8, 1997, which is incorporated herein by reference. A general discussion of watermarking may be found in Jian Zhao, "Look, It's Not There", in: BYTE Magazine, January, 1997.
While each of the categories of techniques for discouraging illegal copying has its strengths, each has its weaknesses. As indicated above, the techniques for keeping copies out of the hands of unauthorized persons are of no help once a digital representation escapes; watermarking by itself cannot hinder illegal copies from being made. What is needed is an improved technique for preventing illegal copying which combines the strengths of the techniques for keeping copies out of the hands of unauthorized persons and the watermarking techniques.